well ask lots of people and you will get differeing ideas and opinions.
i think some mature water can help if excessive w/c being done as it contains some microbes that are good for the fish - but yes does not populate the filter with bacteria but does feed it. taking gunk from another filter will help to introduce bacteria which will hopefully stick in your filters a bit, there is no perfect option but sounds like you are doing the right things.
might help if you can post a pic of thetank so we can get a feel of it better?
i think some mature water can help if excessive w/c being done as it contains some microbes that are good for the fish - but yes does not populate the filter with bacteria but does feed it. taking gunk from another filter will help to introduce bacteria which will hopefully stick in your filters a bit, there is no perfect option but sounds like you are doing the right things.
might help if you can post a pic of thetank so we can get a feel of it better?